The present invention relates to a dielectric fluid for use in electrical devices. More particularly, the present invention relates to a dielectric fluid for electrical capacitors.
Electrical capacitors are used in a variety of applications. Capacitors are generally referred to as large capacitors and small capacitors. Large capacitors include high voltage (greater than 600 volts AC) and low voltage power capacitors, induction heating capacitors, and power factor correction capacitors. Small capacitors are typically found in application categories, such as motor start and run capacitors and lighting capacitors.
One common configuration for electrical capacitors includes a spiral-wound configuration with a pair of capacitor roll sections. Each of the capacitor roll sections is typically a strip of polypropylene having an aluminum or zinc coating. Suitable electrical leads are attached to the spiral-wound roll using conventionally known techniques. The spiral-wound roll is then placed in a casing and the casing is filled with a capacitor fluid.
The capacitor fluids should have a high dielectric constant, maintain a low dissipation factor, and be compatible with other materials in the capacitor. The capacitor fluids must also withstand elevated and fluctuating temperature, pressure, and voltage stress conditions to provide the capacitor with a long operative life.
The capacitor fluid should also be relatively viscous to minimize loss of the capacitor fluid from the capacitor if the casing of the capacitor develops a leak. Additionally, components used in fabricating the capacitor fluid should be biodegradable to minimize environmental damage if the capacitor fluid leaks from the capacitor.
Cichanowski, U.S. Pat. No. 4,388,669, which is assigned to the assignee of the present application, describes polypropylene glycol dielectric fluids. The polypropylene glycol dielectric fluids exhibit high clearability by virtue of low carbon to hydrogen ratio and high weight percentage of molecular oxygen. The polypropylene glycol dielectric fluids also display low swelling effect on the polypropylene film and minimal penetration into the capacitor roll. One drawback of the polypropylene glycol dielectric fluid is that it can leak from a breached capacitor because the polypropylene glycol dielectric fluids possess only moderate viscosities in the range of 500 centipoise.
Bentley, U.S. Pat. No. 4,656,558, discloses a dielectric liquid containing polybutene with an average molecular weight of at least 800. Bentley, U.S. Pat. No. 4,787,010, describes forming a dielectric material from a mixture of polybutene and polyethylene. Bentley ""010 indicates that the dielectric material is semi-solid to prevent the dielectric material from escaping from a capacitor in which the dielectric material is used. Using dielectric materials formed from polybutene present problems in obtaining desirable clearance results because polybutene lacks molecular oxygen.
A dielectric fluid for use in a capacitor. The dielectric fluid contains a mixture of vegetable oil, transformer oil, and a clay material. The dielectric fluid exhibits a thixotropic index of greater than 20. Advantageously, the dielectric fluid exhibits a dielectric strength of greater than 32 kilovolts. The dielectric fluid also exhibits minimal degradation of components when used in a capacitor.
While the present invention is susceptible of embodiments in various forms, there is shown in the drawings and will hereinafter be described presently preferred embodiments with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiments illustrated.
A dielectric fluid produced according to the present invention generally includes a mixture of vegetable oil, transformer oil, and a clay material. The present invention also relates to a capacitor that includes a casing, a capacitor roll, and dielectric fluid produced according to the present invention.
The dielectric fluid exhibits a high thixotropic index, which enables the capacitor to be readily filled with the dielectric fluid and minimizes the potential for leakage of the dielectric fluid from the capacitor. As used herein, the term thixotropic index is a ratio of the dielectric fluid viscosity at a low shear rate to dielectric fluid viscosity at a high shear rate.
The dielectric fluid exhibits high clearability because of the fluid""s low carbon to hydrogen ratio and the high weight percentage of molecular oxygen. The dielectric fluid of the present invention has a low swelling effect on polypropylene film and thereby minimizes capacitance losses due to fluid interaction. Additionally, the dielectric fluid has minimal penetration into the capacitor roll because of the dielectric material""s high surface tension.
The vegetable oil is a low viscosity, hydrocarbon-based oil. The vegetable oil is selected with a high temperature and pressure stability. The vegetable oil is also selected to be substantially inert with respect to the other components in which the dielectric fluid is used. For example, the vegetable oil does not react with the polypropylene film used in fabricating the other portions of the capacitor.
The vegetable oil is preferably soybean oil. A preferred vegetable oil may be obtained from Hunt-Wesson, Inc. of Fullerton, Calif. However, a person of ordinary skill in the art will appreciate that it is possible to use one or more other vegetable-based oils alone or in conjunction with soybean oil without departing from the present invention.
The concentration of the vegetable oil in the mixture is between about 40 and 85 percent and desirably between about 40 and 60 percent. Preferably, the concentration of the vegetable oil in the mixture is approximately 50 percent. As used herein, all references to percent are percent by weight unless indicated otherwise.
The transformer oil is preferably an insulating mineral oil. The transformer oil is selected with a high temperature and pressure stability. While the transformer oil is typically more aggressive towards the other components in the capacitor than the vegetable oil, the transformer oil is selected to minimize degradation of the other components in which the dielectric fluid is used.
A preferred light mineral insulating oil may be obtained from Exxon Corporation of Houston, Tex., under the designation UNIVOLT 60 mineral oil. A person of ordinary skill in the art will appreciate that it is possible to use one or more other mineral oils in conjunction with light mineral insulating oil without departing from the present invention.
The concentration of the transformer oil in the mixture is between about 15 and 60 percent and desirably between about 40 and 50 percent. Preferably, the concentration of the transformer oil in the mixture is approximately 48.5 percent.
The clay material is incorporated into the mixture to raise the viscosity of the mixture. The clay material is preferably in the form of a finely divided powder. Depending on the clay material selected, it may be necessary to use an activator in conjunction with the clay material to produce a desired level of viscosity enhancement.
A suitable clay material for use in the present invention is montmorillonite clay. A preferred montmorillonite clay is marketed under the designation CLAYTONE 40 by Southern Clay Products, Inc., of Gonzales, Tex. Another suitable clay material for use in the present invention is bentonite clay. A preferred bentonite clay is marketed under the designation BENTONE 1000 by RHEOX, Inc., of Hightstown, N.J.
The concentration of the clay material in the mixture is up to about 6 percent and desirably between about 2 and 5 percent. Preferably, the concentration of the clay material in the mixture is approximately 3 percent.
An activator is preferably used in conjunction with the montmorillonite clay. The activator enhances the dispersion of the clay material in the mixture of vegetable oil and transformer oil. The activator also reduces settling of the clay material in the capacitor fluid. It is believed that the activator produces these results by binding to the surface of the clay material.
A suitable activator for use with the present invention is polyethylene-polypropylene copolymer. A preferred polyethylene-polypropylene copolymer is marketed under the designation PLURONIC 31R1 block copolymer surfactant by BASF Corporation of Parsippany, N.J. Another suitable activator for use in the present invention is a tetrafunctional block copolymer of poly-oxypropylene/oxyethylene, based on ethylene diamine, which is marketed under the designation TETRONIC 150R1 by BASF Corporation of Parsippany, N.J.
Yet another activator for use in the present invention is propylene carbonate, which is marketed by Arco Chemical Company of Newtown Square, Pa., under the designation ARCONATE 1000.
The concentration of the activator used in formulating the dielectric fluid of the present invention is up to 3 percent. The concentration of the activator is preferably selected based on the concentration of the clay material. A ratio of the clay material to activator is preferably between 3:1 and 6:1. A preferred concentration of the activator in the dielectric fluid is approximately 0.5 percent.
The dielectric fluid is preferably prepared by mixing the transformer oil, the clay material, and the activator, if used. To ensure a homogenous mixture is produced, the mixing is continued for approximately 1 hour. Vegetable oil is then added to the mixture and mixing is continued for an additional hour.
The dielectric fluid exhibits a thixotropic index of at least 20 and desirably between about 20 and 500. Preferably, the thixotropic index is between about 50 and 200. As noted above, the thixotropic index is a ratio of the dielectric fluid viscosity at a low shear rate to the dielectric fluid viscosity at a high shear rate.
For purposes of demonstration, the low shear rate viscosity is preferably obtained using a spindle rotation rate of approximately 0.6 revolutions per minute. The viscosity is preferably measured in a Brookfield viscometer. The high shear rate viscosity is preferably obtained using a spindle rotation rate of approximately 60 revolutions per minute.
The low shear rate viscosity is typically at least 500 centipoise and desirably at least 10,000 centipoise. Preferably, the low shear rate viscosity is between about 15,000 centipoise and 75,000 centipoise. The high shear rate viscosity is typically less than 500 centipoise. Preferably, the high shear rate viscosity is between about 100 centipoise and 500 centipoise.
Because the dielectric fluid exhibits a high thixotropic index, the dielectric fluid readily flows into the capacitor and fills the voids between the capacitor roll sections. The high thixotropic index of the dielectric fluid also reduces the potential for leakage of the dielectric fluid from the capacitor because the dielectric fluid exhibits a high viscosity once placed in the capacitor.